Known in the art is a method of potentiometric measurement of microconcentrations of gases in the air and an apparatus for carrying out the method (SU, A, 976368). The apparatus comprises an electrode cell containing an electrolyte, an indicator electrode and a comparison electrode, an amplifier for determining a signal corresponding to hydrogen cyanide vapour content, and a recorder.
This apparatus cannot ensure the desired accuracy of measurement of hydrogen cyanide vapour content because of a gradual accumulation of cyanides in the electrolyte and a drift of the electrode function. It is also necessary to keep an eye on the electrolyte level in the electrolyte cell since a deviation of this level even by 1-2 mm from the pre-set value causes a disruption in operation of the apparatus.
Also known in the art is an apparatus for measuring hydrogen cyanide vapour content (GB, A 2074323), comprising an indicator electrode in the form of an ionselective electrode sensitive to cyanide ions, and a comparison electrode, each electrode having a sensor member, a salt bridge through which the indicator electrode is connected to the comparison electrode, a sealed electrode cell accommodating the indicator electrode having ports for admission and discharge of the air to be tested, and a means for supplying the electrolyte to the indicator electrode. The apparatus also has an amplifier for determining a signal corresponding to hydrogen cyanide vapour content, a compressor for supplying a continuous flow of the air to be tested to the electrode cell, and the means for supplying the electrolyte to the indicator electrode comprising a small-diameter tube open at one end, having its open end directed at the sensor member of the indicator electrode.
This apparatus has a number of disadvantages:
low speed because it takes much time to achieve a full change of an air batch in the electrode cell and because of adsorption of hydrogen cyanide vapour by the walls of the electrode cell, and a long time is needed for cyanide diffusion through a relatively thick electrolyte film (which is determined by surface tension of electrolyte) on the surface of the sensor member of the indicator electrode;
low accuracy of the analysis which is caused by instability of the electrolyte film under washing with electrolyte and by a drift of the electrode function of the indicator electrode;
low sensitivity which is caused by a relatively low partial pressure of hydrogen cyanide vapour over the electrolyte film on the surface of the sensor member of the indicator electrode and a certain loss of sensitivity because of chloride ions penetration from the salt bridge (when a single salt bridge is used); and
difficulties in operation since frequent calibrations (once every 24 hours) are necessary, the electrolyte should be frequently prepared and poured (10 1 in 2 to 15 days), and the salt bridges should also be maintained.